Why do you think:
the Leader of the Islamic Revolution of Iran is addressing European and American youths?

I think Ayatollah Khamenei realizes that young people are more open to new information than older people. Also, young people use alternative media more than older people and are more aware that mainstream media are untrustworthy. Since the mainstream media will block or distort his message, he is going around the censorship by speaking directly to Western young people.

Why did the leader invite them to receive and gain Islam through its primary sources like the Quran and teachings and humane doctrine of Prophet Mohammad?

I think he is asking young people to recognize that much of the information about Islam in the mainstream media, and to a lesser extent in the Western educational system, is propaganda. To gain a better understanding, young people (and others) should bypass the middleman and get their information from the primary sources.

What are the main obstacles/hurdles on Western countries' path to know true Islam? Why do the political leaders in the west try to introduce Islam as the west's great enemy?

Of the three major Western monotheisms, Islam is most compatible with reason and scientific approaches to knowledge. But it is not easy for Western people to grasp this. One obstacle is the translations. Most translations of Islamic material in Western languages are either unfriendly (the non-Muslim ones) or clumsy (the Muslim ones). Another obstacle is that the Saudis have spent a lot of money spreading their Wahhabi version of Islam, which Western people generally find unappealing. Most Western people tend to like the more mystical and sophisticated approaches of Sufism and (potentially) Shia Islam.
But the biggest hurdle is Zionist domination of Western media. The Zionists are basically at war with Islam, so they portray Islam in a negative light.

About Me

I'm the author of Questioning the War on Terror: A Primer for Obama Voters; Truth Jihad; A Guide to Mysterious San Francisco; and editor of 9/11 and American Empire v.2, and have taught Arabic, Islamic Studies, French, American Civilization and others subjects. Widely regarded as the world's leading Muslim 9/11 truth activist (for what that's worth) I spent 2006-2008 as the only talk show host featured on all three leading patriot radio networks (GCN, RBN, and WTPRN). I ran for Congress in Wisconsin's 3rd District in 2008: http://www.barrettforcongress.us
Currently I'm working on a couple of book projects...